Bonded Phase Silica Gels

Whatever steps in the manufacturing process, the final product has to be reproducible as mentioned. When bonded silica gel layers started to be produced, more variables become important during their manufacture. There was also the consideration of scale of the manufacturing process. If making a bonded phase sorbent for HPLC columns, the batch size need be only 1-2 kg to fill a few hundred HPLC columns the batch size for bonded sorbents for coating thin layers may be 10 or 20 times this amount. 

Normally, few problems will be encountered with the use of the TLC or HPTLC products, but the chromatographer should routinely record any batch numbers on the packing of that product to allow back tracking with the manufacturer should this be 

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B. W. Sands, Y. S. Kim, and J. L. Bass, Characterization of bonded-phase silica gels with different pore diameters, J. Chromatogr., 360 353 (1986). 

Bondesil. [Analydchem Inf I.] Bonded phase silica gels. 

Silica gel may be chemically modified in a number of ways that alter both its chromatographic and physical properties. As shown in Fig. 4, the reactive silanol groups of silica gel may be blocked with a variety of silylchlorides to produce a nonpolar (reverse phase) or polar (bonded normal phase) chromatography support. The most commonly employed bonded phase silica gels are the reverse phase class although the use of bonded normal phase silica has increased with improved materials. 

Polymeric resins have several advantages over the commonly used silica-gel based reverse phase materials. The cost of polystyrene-based resins is considerably less than bonded phase silica gel adsorbents. Also, the potential... 

The analytes listed in Tables 6.12 and 6.13 are flat, so steric hindrance can be neglected when studying their molecular interactions, therefore, a simple and homogeneous model was used instead of a brush-type bonded phase silica gel model whose blush types and density were designed to reduce the steric hindrance effects of the analytes. A model butyl phase with adsorbed naphthalene is shown in Figure 6.34. 

In LPLC, a mobile phase is allowed to flow through a densely packed sorbent. The separation mechanism is adsorption or size-exclusion depending on the choice of packing material for the stationary phase (adsorption silica gel, bonded-phase silica gel, alumina, polystyrene size-exclusion polyacrylamide, carbohydrates). This is almost similar to... 

The chromatographic resolution of bi-naphthol enantiomers was considered for simulation purposes [18]. The chiral stationary phase is 3,5-dinitrobenzoyl phenyl-glycine bonded to silica gel and a mixture of 72 28 (v/v) heptane/isopropanol was used as eluent. The adsorption equilibrium isotherms, measured at 25 °C, are of bi-Langmuir type and were proposed by the Separex group ... 

The separation of bi-naphthol enantiomers can be performed using a Pirkle-type stationary phase, the 3,5-dinitrobenzoyl phenylglycine covalently bonded to silica gel. Eight columns (105 mm length) were packed with particle diameter of 25 0 fiva. The solvent is a 72 28 (v/v) heptane isopropanol mixture. The feed concentration is 2.9 g for each enantiomer. The adsorption equilibrium isotherms were determined by the Separex group and already reported in Equation (28) [33]. 

Many types of chiral stationary phase are available. Pirkle columns contain a silica support with bonded aminopropyl groups used to bind a derivative of D-phenyl-glycine. These phases are relatively unstable and the selectivity coefficient is close to one. More recently, chiral separations have been performed on optically active resins or cyclodextrins (oligosaccharides) bonded to silica gel through a small hydrocarbon chain linker (Fig. 3.11). These cyclodextrins possess an internal cavity that is hydro-phobic while the external part is hydrophilic. These molecules allow the selective inclusion of a great variety of compounds that can form diastereoisomers at the surface of the chiral phase leading to reversible complexes. 

In addition to the common use of vancomycin and teicoplanin, the use of other antibiotics as chiral selectors in HPLC were limited. Only a few reports are available in the literature dealing with these antibiotics as HPLC CSPs. Armstrong et al. [42] resolved about 230 racemates on the ristocetin A antibiotic covalently bonded to silica gel. The resolution was carried out using the three modes of mobile phases. The results were complimentary to those obtained on vancomycin and teicoplanin CSPs. In another study, the effect of selector coverage and mobile phase composition on enantiomeric resolution with ristocetin A CSP was carried out by Armstrong et al. [43]. Thiostrepton-based CSP was also used to resolve the enantiomers of thioridazine, 2,2,2-trifluoro-l-... 

The use of antibiotic-based CSPs has been reported in capillary electrochromatography (CEC) for chiral resolution [60]. Teicoplanin CSP covalently bonded to silica gel was used to resolve the enantiomers of tryptophan and dinitrobenzoyl leucine by CEC [61]. Good levels of enantioselectivity were obtained with optimized separations. Vancomycin covalently bonded to silica gel was also evaluated in CEC for the chiral resolution of thalidomide and jS-adrenergic blocking agents under all the three mobile phase modes. The... 

It is well known that the chiral resolution of these CSPs occurred as a result of the exchange of ligands and enantiomers on the same metal ion. Therefore, these CSPs are suitable only for those racemates which can coordinate with the metal ion. Therefore, racemates like amino acids, amines, and hydroxy acids have been resolved successliilly by the ligand-exchange process. As mentioned earlier, either the individual chiral ligand or one complexed with a metal ion is bonded onto silica gel support. Therefore, in the case of the first type of CSP, the metal ion is used in the mobile phase no metal ion is required in the mobile phase in the latter case. 

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